Our first experiment was the structure determination of a cytoplasmic domain from a voltage-gated potassium channel. The domain contains 140 amino acids and controls the gating of a potassium channel that is responsible for electrical conduction in the heart. Synchrotron radiation was required because of the microscopic size of the crystals (50 microns) and because of severe nonisomorphism upon heavy atom derivitization. Protein containing selenomethionine was produced and the structure was solved in a 3 wavelength MAD experiment at F2 with data to 2.6 [unreadable]. The structure directly led to a hypothesis for the mechanism by which the domain regulates gating of the channel. The hypothesis has been tested through functional studies over the eight months since the structure determination and a paper is being written. Our second experiment was the structure determination of an integral membrane potassium channel using the A1 facility. The structure was solved by MIR with data to 3.2 [unreadable] and required numerous data collection sessions for both native and derivative sets. This structure forms the basis for our current understanding of potassium selectivity in ion channels. 4.